1. Field of the Invention
The present invention relates to a high-accuracy, variable-reluctance (VR) resolver whose shaft angle multiplier is 1X and which is used for, for example, measurement or control of a rotational angle or position.
2. Description of the Related Art
A variable-reluctance (VR) resolver, which includes a stator having an excitation coil and output coils wound around its magnetic poles, and a rotor having an arbitrary salient pole shape, outputs a resolver signal, which is a two-phase voltage signal including a sin voltage signal and a cos voltage signal, which vary with a rotational angle of the rotor. Such a VR resolver must output a resolver signal whose shaft angle multiplier is 1X (hereinafter referred to as “1X resolver signal”) and which serves as a reference for detection of an absolute position.
A VR resolver whose shaft angle multiplier is 1X (hereinafter referred to as “1X VR resolver”) is formed from a set consisting of a stator stack and a rotor stack having an eccentric shape. In the case where the stator and the rotor are assembled in a misaligned state; i.e., the center axis of the rotor is deviated from the center axis of the stator, output voltage signals, which vary with the rotational angle of the rotor, greatly deteriorate in accuracy as compared with the designed output voltage signals.
The accuracy deterioration occurs for the following reason. In the case where the shaft angle multiplier of a resolver is 1X, the shape of the salient pole is determined to have a single peak within a single rotation (mechanical angle: 360 degrees) of a rotary shaft. Therefore, the change in radius of the salient pole per unit rotational angle becomes small, and thus, the amounts of change in the output voltage signals per unit rotational angle become small. Accordingly, even a small center deviation between the stator and the rotor produces large errors in the output signal voltages.
Meanwhile, machining accuracy may be increased in order to manufacture a high-accuracy 1X VR resolver. However, the machining accuracy can be increased only within a limited range. For example, it is said that the accuracy of such a 1X VR resolver can be increased only to a level of about ±2°. Further, increased accuracy inevitably results in an increase in cost, and makes mass production difficult.
Conventionally, an absolute-position detection apparatus which can solve the above-described problem has been proposed (see, for example, Japanese Patent Application Laid-Open (kokai) No. H03-148014). The absolute-position detection apparatus utilizes, in combination, a VR resolver whose shaft angle multiplier is 1X and in which the phase of a detection signal changes by 360 degrees when the rotary shaft rotates one turn (hereinafter referred to as “1X VR resolver”) and a VR resolver whose shaft angle multiplier is NX and in which the phase of a detection signal changes by 360 degrees every time the rotary shaft rotates a 1/N turn (hereinafter referred to as “NX VR resolver”), where N is an arbitrary integer. In the apparatus, the 1X VR resolver detects a pole corresponding to the resolution (1/N turn), and the rotational angle position within the detected pole (an area corresponding to 1/N turn) is calculated on the basis of the detection signal from the NX VR resolver.
The term “shaft angle multiplier” refers to the ratio of an output electrical angle θe of a VR resolver to an actual input mechanical angle θm of the resolver, and in general, the mechanical angle θm is obtained through division of the output electrical angle θe by the shaft angle multiplier.
Even in the conventional apparatus which uses a 1X VR resolver and an NX VR resolver in combination, a detection signal output from the 1X VR resolver is still used as a reference. When a characteristic curve of a 1X digital signal obtained through R/D conversion (Resolver to Digital conversion) of the output voltage signal of the 1X VR resolver is drawn, its slope is small. This means that a small variation in input causes a large variation in output, so that output errors are easily generated. Moreover, when the shaft angle multiplier N of the NX VR resolver is increased (the number of salient poles is increased), a correct pole on the characteristic chart for the NX resolver may fail to be selected if the output of the 1X VR resolver does not change linearly, because of influence of errors. Therefore, a conventional apparatus cannot solve the problem involved in the conventional 1X VR resolver such that the detection signal is very likely to be influenced by errors.
Further, the conventional apparatus has a problem of increased size, because both the 1X VR resolver and the NX VR resolver are incorporated in the apparatus. Moreover, since the VR resolvers are connected together via a speed reduction mechanism, the conventional apparatus generates mechanical vibration and noise, and causes operational malfunctions as a result of wear.
There has been known another absolute-position detection apparatus which uses two VR resolvers; i.e., an NX VR resolver and an (N+1)X VR resolver, and which obtains a difference between saw tooth signals output from the VR resolvers to thereby produce a 1X resolver signal which continues over a predetermined period (see, for example, Japanese Patent Application Laid-Open (kokai) No. S60-152251).
However, since the above apparatus uses two VR resolvers, a task of adjusting their mutual relationship is required, and the overall size of the apparatus increases. In addition, the apparatus requires calculation processing means for converting the signals output from the NX VR resolver and the (N+1)X VR resolver to the 1X resolver signal, which results in an increase in cost, and therefore, the apparatus is not practical.
In order to reduce size, there has been proposed a position detection apparatus which employs a structure in which two resolver units each consisting of a stator and a rotor are provided in tandem.
Such a position detection apparatus including two resolver units (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2001-183169) includes a first position detection section which outputs a first position detection signal that periodically changes P times per single revolution, and a second position detection section which outputs a second position detection signal that periodically changes (P−N) times per single revolution. The first and second position detection sections are disposed on a common rotation shaft. On the basis of the first and second position detection signals, first and second calculation sections output electrical angle signals of saw tooth shape which periodically change P times and (P−N) times, respectively, per single revolution. A third calculation section calculates a difference between the signals output from the two calculation sections. When the calculated difference is positive, the calculated difference is used as is. When the calculated difference is negative, an electrical angle of 360 degrees is added to the calculation result. Thus, the apparatus outputs a position detection signal which periodically changes N times per single revolution.
When a 1X resolver signal is obtained by making use of the apparatus disclosed in Japanese Patent Application Laid-Open (kokai) No. 2001-183169, the values of P and N are set such that the value of P−N is 1. When the values of P and N are increased, higher mechanical accuracy is required of, for example, magnetic pole teeth. Moreover, since the saw tooth signals are compared with each other at each timing, higher measurement accuracy is required, inducing the degrees of their inclinations. Therefore, this apparatus is also impractical.